This invention relates to a method of producing an active matrix array using thin film transistors for a liquid-crystal display device.
Thin film transistors (TFT) using thin films of amorphous semiconductors such as amorphous silicon (a-Si) can have a large area and can be produced at low temperatures, and can be fabricated using inexpensive substrates such as glass. For these reasons, they are the most suitable for making inexpensive liquid-crystal devices of large areas. When used in a liquid-crystal display device, however, TFTs must be shielded from light because a-Si is sensitive to light which changes its resistivity. Shielding means must be provided not only for light incident from the front, but also for light incident from the rear when a transmission type of liquid-crystal display device is used, and hence the number of steps required to produce such a display increases. When shielding films are provided on the front and back of a TFT, for example, the production thereof requires the following seven masking steps:
(1) lower shielding film
(2) gate electrode wiring
(3) a-Si
(4) contacts
(5) source-drain metal electrode wiring
(6) upper shielding film
(7) formation of holes of the surface protective insulator film
In addition:
(8) display electrodes are necessary when this TFT is used in a liquid-crystal display device, and
(9) when a signal-holding capacitor is incorporated in each picture element, a masking step for the common electrodes is also necessary.
Thus the production of a TFT array needs eight or nine masking steps, so that the economy of using TFTs is compromised, and the increased number of production steps results in a drop in production yield.